Drilling mud is used while drilling oil and natural gas wells on a drilling rig to control subsurface pressures, lubricate the drill bit, stabilize the wellbore, and carry cuttings and other debris to the surface, among other functions. The drilling mud is pumped through a drill string and out of a drill bit, then returned to the surface through the annular space between the drill string and the wellbore. At the surface, cuttings and other solids entrained in the mud are typically separated from the mud by a vibrating screen separator called a shale shaker.
Shale shakers include a lower stationary base, called an underflow pan, and an upper vibratable basket that is connected to a high frequency linear angular motion or rotary vibratory motor. Mounted generally horizontally in the basket is a screen deck comprising one or more screen assemblies having one or more layers of tensioned screening material. In use, solids-laden mud flows onto one end of the screen deck, and solids retained by the screening material are conveyed by the vibratory action of the motor to the opposite end where they are discharged. Fluids and the smallest particles pass through the screening material and are collected in a tank system for further processing and reuse in the active system.
Screen assemblies include tensioned screening material attached to a rigid perforated plate, which is in turn attached to a generally rectangular structural frame. The perforated plate supports the screening material and contains a plurality of openings to permit the screened fluid to pass through the screening assembly. The frame is used to mount the screen assembly onto corresponding support slides of the basket and the sides of the screening assembly are secured thereto. For example, wedges can be wedged between the top periphery of the screen assembly and wedge supports extending inwardly from sides of the screen basket. The sides of the screening assembly are thereby securely sandwiched between the support slides and the wedges.
Typically, at least one deck of screen assemblies is provided. Sometimes a second, partial upper deck of coarse screens is provided, sometimes referred to as a scalping deck. Scalping decks are heavy because they include a full length tray thereunder which captures mud passing therethrough. The increased weight necessitates greater vibratory force from the motors in order to achieve the desired level of separation of solids from the drilling mud, which in turn can result in shortened screen life. Further, the screens are narrower than the apparatus width, which applicant understands may be due to weight considerations or to avoid mistaken placement of fine mesh lower screens in the upper deck locations. Further, due to the weight, vibratory effectiveness is reduced. Some operators remove the scalping deck in its entirety.
When dual decks are used, the narrow screens of the scalping deck, the increased weight, and other design factors lessen the effectiveness of the current vibrating apparatus.
There is a need in the art for an improved vibratory separator, including a separator that has an improved scalping deck.